1. Field of the Invention
The present invention relates to a photovoltaic device, and more particularly, it relates to a photovoltaic device having a heterojunction which is formed by combining crystalline and amorphous semiconductor materials with each other.
2. Description of the Background Art
In recent years, active studies have been made on a photovoltaic solar cell device which is prepared from polycrystalline silicon. In particular, a solar cell having a heterojunction formed by combining amorphous and polycrystalline silicon materials with each other is receiving interest in view of its low cost and high conversion efficiency.
A plurality of such photovoltaic devices are generally connected in series with each other, in order to attain a sufficient voltage. A structure obtained by connecting such a plurality of photovoltaic devices in series with each other is generally called a module. Tabs are employed for connecting collector electrodes of the photovoltaic devices with each other by soldering, thereby modularizing the devices.
When a plurality of photovoltaic devices are electrically connected with each other by such tabs, however, certain characteristics of these devices may be disadvantageously changed by heat which is applied for soldering. Namely, amorphous semiconductor layers may be crystallized by heating thereby causing cracking, for example, leading to penetration of the metal that forms the collector electrodes and the like through the resulting cracks. Consequently, the cells may be shorted thereby damaging or disrupting p-n junctions therein.
While a photovoltaic device is generally provided with a collector electrode in a final step, it not possible to apply ordinary hot sintering silver paste to a hetero-junction type photovoltaic device having an amorphous silicon layer, which is inferior in heat resistance. In such a photovoltaic device having an amorphous silicon layer, therefore, a collector electrode is with through low-temperature setting silver paste or by patterning of deposited silver. However, the low-temperature setting silver paste has disadvantages such as a high resistance and inferior characteristics. When the collector electrode is formed by patterning of deposited silver, on the other hand, the cost is increased since the number of steps is increased and a number of materials are required.